This is untrue in languages with proper optimizing compilers, designed for performance, e.g. C++, Rust, Zig. A functional iterator transformation chain is often just the same performance as a "traditional" for loop.

But try that in Java and you'll often see a 3x-10x difference, especially if you run into problems with boxing. Streams are known to be notoriously slower than for loops with index. I once have even doubled the speed of a for loop by unrolling it manually (Java 8). Not all compilers are equally strong at optimization.

Anyway, asking such questions in an interview for a junior/regular developer is just a distraction.